Table of contents : Picard-Vessiot rings....Pages 4-27 Algorithms for difference equations....Pages 28-34 The inverse problem for difference equations....Pages 35-44 The ring S of sequences....Pages 45-51 An excursion in positive characteristic....Pages 52-59 Difference modules over $$\mathcal{P}$$ ....Pages 60-67 Classification and canonical forms....Pages 71-76 Semi-regular difference equations....Pages 77-94 Mild difference equations....Pages 95-110 Examples of equations and galois groups....Pages 111-126 Wild difference equations....Pages 127-148 q -difference equations....Pages 149-174